This invention relates to an oscillator assembly for providing a constant frequency output over a range of temperatures.
Oscillators, specifically piezoelectric oscillators, have sections which are temperature sensitive. Such oscillators exhibit a variation in the output frequency when the temperature in which the temperature sensitive sections operate varies. For many environments, such as in military applications or scientific instrumentation, a very precise frequency is required over a range of temperatures. The variation in frequency output with temperature is called the oscillator's temperature characteristic.
One way to compensate for variations in frequency output for given differences in temperature is to provide a compensating circuit. Compensating circuits provide feedback to the oscillator during operation. The feedback varies in proportion to the temperature of the surrounding environment. Compensating circuits typically comprise a thermistor-resistor network driving a varactor diode in series with the oscillator. This thermistor-resistor network generates a voltage which varies as a function of temperature. This voltage, when applied to the varactor diode, shifts the output frequency of the oscillator. If the elements of the thermistor-resistor network are chosen properly, the output shift caused by the varactor diode will approximately cancel the oscillator's frequency variation over a range of temperatures.
However, the attempted compensation described above is never perfect. It is typical for there to be a frequency error which limits the accuracy of the oscillator at certain temperatures. When an oscillator is required to operate in an environment having large temperature variations, a compensated oscillator as described above may be unsatisfactory.
An alternative to this approach of compensating for the temperature effects with a compensating circuit is to maintain the temperature of the temperature sensitive section of an oscillator at a constant temperature. In order to maintain the temperature of an oscillator's temperature sensitive section at a constant temperature, temperature sensitive section is preferably maintained in an oven at a temperature above any temperature possible in the operating environment. In this way, a very accurate error-free frequency is produced for a wide range of environmental conditions. However, the power consumption of the resultant oscillator assembly is a critical factor. It is desirable to minimize power consumption. To maintain a high constant oscillator section temperature, a large amount of power is consumed by the oscillator assembly.
The oven temperature should be set to a value comfortably above the highest ambient temperature the oscillator is expected to encounter. For military applications a high ambient temperature of up to 85.degree. C. is common and the oscillator circuit temperature is regulated to be about 100.degree. C. On the other hand, a low limit on ambient temperature for military service is about -55.degree. C. and the amount of power needed to maintain the oven at 100.degree. C. with a -55.degree. C. ambient temperature is often too high to allow use of an oven in portable battery powered applications.
In order to minimize the power consumption it is essential to insulate the temperature sensitive oscillator section from excessive heat loss. Solutions which have been suggested for insulating the temperature sensitive oscillator section are enclosing the oscillator section in a dewary-type flask, enclosing the oscillator section in a vacuum envelope and use of plastic foams surrounding the temperature sensitive oscillator section.
A dewar-type flask has a double wall construction wherein the space between the walls is evacuated. Dewar-type flasks having low loss have to be made of very thin glass and are thus often too fragile for rugged use. In addition, thermal losses through the open end of the dewar-type flask tend to be large.
A vacuum envelope completely surrounding the temperature sensitive oscillator section minimizes thermal conduction and convection losses. This is a costly technique, however, since high vacuum technology must be employed. Anything that would permit or cause the gas pressure in the envelope to rise above 10.sup.-4 torr would quickly destroy the insulating properties of the vacuum envelope. Thus, materials chosen for the oscillator assembly must be of low vapor pressure and outgassed. Outgassing refers to the property of materials to give off various gases over the life of the material. To outgas a material, it is necessary to try to force as much of the gas as possible out of the material before using the material. There are also other manufacturing and assembly problems, such as the very stringent requirements of leak rate into the vacuum.
Plastic foam insulation is a low cost technique but does not provide sufficient insulation to reduce the required power to levels now desired and is subject to aging effects as gas seeps in or out of the foam.